1. Field of the Invention
Embodiments of the present invention generally relate to apparatus and methods for producing polymer. More particularly, embodiments of the present invention relate to apparatus and methods for extruding polyolefin using high speed, directly driven rotating equipment.
2. Description of the Related Art
Continuous mixers are the mainstay of many commercial polyolefin pelletizing production lines. A continuous mixer is different from an extruder in that continuous mixers are generally used only to melt and mix the resin, while extruders further provide pressure generation to force the molten resin through a die. Like extruders, continuous mixers can have one or multiple screws.
General features that distinguish various twin-screw mixers from other mixers are the relative direction of rotation and the degree of intermeshing of the rotors. Twin-screw mixers having rotors that rotate in the same direction are co-rotating twin-screw mixers. When the rotors rotate in opposite directions the machine is a counter-rotating mixer. The degree of intermeshing can vary from fully intermeshing to partially intermeshing to non-intermeshing, or tangential. Continuous mixers with rotating screws fully supported by external, oil lubricated bearings can be considered a more reliable and robust mixer compared to twin-screw extruders having rotating screws supported by the twin-screw extruder barrel(s) and lubricated by the polymer.
Continuous mixers are especially common for polyolefin production, and are typically used in conjunction with gear pumps to reach pressure requirements for forcing the molten polymer through the die. As such, the rates through the mixer and the gear pump must be compatible. Typically, gear reducers are attached to the mixer motor and/or gear pump motor to control the respective motor speeds and throughput so that the machines can operate at cooperative rates. See, for example, U.S. Pat. Nos. 4,032,391; 4,336,213; 4,452,750; 5,458,474; and 5,728,335. Such additional components are expensive, require maintenance time and expense, consume energy, and often lack in efficiency. Also, such components have large space requirements within the fabrication facility.
Direct driven extruders have been described in the literature. See, for example, U.S. Pat. Nos. 4,249,877; 4,741,264; and 2003/0012838 A1. However, direct driven continuous mixers have not been introduced nor have pelletizing systems that utilize direct driven mixers and gear pumps. As mentioned above, designers of continuous mixers are faced with all different design criteria and operating requirements than extruders. There is a need, therefore, for a reliable continuous mixer and gear pump that can operate without gear reduction to decrease capital, maintenance and operating costs.